Abstract
IC fabs inspect critical masks on a regular basis to ensure high wafer yields. These requalification
inspections are costly for many reasons including the capital equipment, system maintenance, and
labor costs. In addition, masks typically remain in the “requal” phase for extended, non-productive
periods of time. The overall “requal” cycle time in which reticles remain non-productive is
challenging to control. Shipping schedules can slip when wafer lots are put on hold until the master
critical layer reticle is returned to production. Unfortunately, substituting backup critical layer
reticles can significantly reduce an otherwise tightly controlled process window adversely affecting
wafer yields.
One major requal cycle time component is the disposition process of mask inspections containing
hundreds of defects. Not only is precious non-productive time extended by reviewing hundreds of
potentially yield-limiting detections, each additional classification increases the risk of manual
review techniques accidentally passing real yield limiting defects. Even assuming all defects of
interest are flagged by operators, how can any person's judgment be confident regarding lithographic
impact of such defects? The time reticles spend away from scanners combined with potential yield
loss due to lithographic uncertainty presents significant cycle time loss and increased production
costs.
Fortunately, a software program has been developed which automates defect classification with
simulated printability measurement greatly reducing requal cycle time and improving overall
disposition accuracy. This product, called ADAS (Auto Defect Analysis System), has been tested in
both engineering and high-volume production environments with very successful results. In this
paper, data is presented supporting significant reduction for costly wafer print checks, improved
inspection area productivity, and minimized risk of misclassified yield limiting defects.
